The Tester in Your Pocket

Working with fiber optics puts some unusual strain on your vision. Experienced techs have learned to deal with it. They wear safety glasses to avoid fiber shards from cleaving fibers. Spotting those tiny glass fibers when splicing or terminating cables can be difficult until you learn to use a black work mat or other dark background that makes them stand out.

One problem persists: seeing the light transmitted in the fiber. Fiber optic systems use infrared light, which is beyond most people’s visual range.

Knowing if light is present in the fiber can be important. Many people worry about the light in the fiber being harmful to eyes. For most systems, especially premises cabling, the light level is too low to be harmful. However, in CATV or wavelength division multiplexing (WDM) systems, the amount of light could be hazardous. One should never look into the end of a fiber with a microscope without knowing if there is power present, since microscopes concentrate all the optical power from the fiber into the eye. I recommend you use microscopes with built-in infrared filters to ensure safety.

Still, it would be convenient to see the light in the fiber and check the output to confirm that the link’s polarity is correct, that is, the transmitters are connected to receivers on the other end of the fiber. If you have properly marked cables, you can use the system’s documentation to check that the connections are correct. Since many systems have incomplete or outdated documentation (if they have it at all), you can verify that you have a signal by visual inspection.

Again, fiber optic systems operate with infrared light at wavelengths outside the range of visible light and beyond the sensitivity of the human eye. For multimode systems operating at 850 nm, some people’s eyes have some sensitivity, but the sensitivity is so low that a signal that looks bright will be quite powerful. At 1,300 nm or 1,550 nm—the wavelengths of light used in single-mode fibers—the eye is completely blind.

In the past, there have been two solutions for checking for power in a fiber: using a fiber optic power meter or a laser detection card. Power meters are the best solution; however, they are costly, and not every technician carries one. The laser detection cards are used with high-powered lasers and are only sensitive enough for the most powerful fiber optics laser systems.

But modern technology has provided an inexpensive, widely available solution: the digital camera. The sensors in digital cameras, both the regular cameras and those in cell phones, are made of silicon and are similar to the detectors used in “short wavelength” fiber optic power meters calibrated for 650 and 850 nm wavelengths.

These digital cameras have good sensitivity to infrared, far greater than the human eye. They are much more sensitive at 850 nm than 1,300 nm, but in tests I’ve witnessed, the sensitivity at 1,300 nm varies considerably from camera to camera. The photo above shows two light-emitting diode (LED) sources of roughly equal optical power at 850 and 1,300 nm photographed with one digital camera to show the differences. As you can see, the 850 nm source is much brighter.

This difference in sensitivity may not be a problem, however, since most 1,300 nm sources are now lasers with considerably higher power levels that are easy to see with the cameras. Few multimode systems use 1,300-nm LEDs anymore since the introduction of 850-nm VCSELs used in all fast networks. Most 1,310-nm lasers used with single-mode fibers have enough power to be seen with a digital camera. The high-powered links used in CATV and dense WDM systems with fiber amplifiers use 1,550-nm lasers that cannot be detected with a cell phone, but if you are working on one of those systems, you better have a fiber optic power meter anyway.

Testing the sensitivity of your cell phone camera or digital camera only takes a minute. Use any test source from a fiber optic test kit, turn the source to maximum power and view it in your camera as shown in the photo above. If you want to be more scientific, couple the source to a patchcord, and use a fiber optic power meter to measure the output power that you are viewing in the camera. In a few minutes, you will have a good idea of the camera’s sensitivity to different power levels.

A video on how to use a cell phone camera in fiber optics is on the FOA channel on YouTube (search for The Fiber Optic Association).

HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.